The light-emitting diode (LED) is a semiconductor solid-state light-emitting device formed by joining a p-type semiconductor and an n-type semiconductor. The LED has been serving as the light source for various kinds of devices because it possesses strong points, such as a long service life, an excellent shock resistance, a lower power consumption and high reliability, and moreover permits a reduction in volume, thickness and light. Particularly, the white LED is now used for warning illuminations which are expected to be reliable, on-vehicle illuminations and liquid crystal backlights which are preferred to promise reduction in volume and weight, and railroad guide plates which are necessitated to ensure visual recognition. It is also expected to find application to general household indoor illuminations.
When an electric current is fed in the forward direction to the p-n junction formed of a direct transition type semiconductor, the electrons and the holes are combined and the light having a peak wavelength corresponding to the forbidden band width of the semiconductor is emitted. Since the emission spectrum of the LED generally has a narrow half-band width of the peak wavelength, the luminescent color of the white LED is acquired exclusively by the principle of the color mixture of light.
For the production of a white color, specifically the following methods have been known.
(1) A method which consists in combining three kinds of LEDs emitting the three primary colors of light, i.e. red color (R), green color (G) and blue color (B), respectively, and mixing the lights of these LEDs.
(2) A method which consists in combining an ultraviolet LED emitting an ultraviolet light and three kinds of fluorescent materials emitting the fluorescences of red color (R), green color (G) and blue color (B), respectively, in response to the excitation by the ultraviolet light and mixing the fluorescences of the three colors emitted from the fluorescent materials.
(3) A method which consists in combining a blue LED emitting a blue light and a fluorescent material emitting a yellow fluorescence having the relationship of a complementary color with the blue light and mixing the blue LED light and the yellow light emitted from the fluorescent material.
A method which obtains a prescribed luminescent color by using a plurality of LEDs necessitates a special circuit for regulating the electric currents of the individual LEDs for the purpose of balancing the relevant colors. In contrast thereto, a method which obtains a prescribed luminescent color by combining an LED and a fluorescent material is at an advantage in obviating the necessity for such a circuit and allowing a reduction in the cost of the LED. Regarding the fluorescent materials of the kind utilizing the LED as a light source, various proposals have been made to date.
For example, a YAG fluorescent material having a YAG-based oxide host crystal represented by the constitutional formula, (Y, Gd)3(Al, Ga)5O12, doped with Ce has been disclosed (refer to Takashi Mukai et al, Applied Physics, Vol. 68, No. 2 (1999), pp. 152-155). This prior art describes the fact that when the first surface of an InGaN-based blue LED chip is coated with a thin layer of a YAG fluorescent material, the blue light emitted from the blue LED and the fluorescence of a peak wavelength of 550 nm emitted from the YAG fluorescent material in response to the excitation by the blue light are mixed to give rise to a white light.
Then, a white LED combining a light-emitting device, such as a nitride-based compound semiconductor capable of emitting an ultraviolet light, and fluorescent materials emitting lights in response to the excitation by the ultraviolet light has been disclosed. As the fluorescent materials to be used herein, a compound (Sr, Ca, Ba)10(PO4)6Cl2:Eu emitting a blue light, a compound 3(Ba, Mg, Mn)O.8Al2O3:Eu emitting a green light and a compound Y2O2S:Eu emitting a red light are disclosed (refer to JP-A 2002-203991).
The existing fluorescent materials generally have a weak point that the spectrum intensity is allayed conspicuously when the excitation wavelength surpasses the near ultraviolet zone.
The white LED obtained by coating the first surface of the InGaN-based blue LED chip with a fluorescent material formed of a YAG-based oxide has been reputed as incurring difficulty in forming a white LED of high luminance because the excitation energy of the YAG-based oxide which is a fluorescent material and the excitation energy of the blue LED as the light source do not coincide and the excitation energy is not converted with high efficiency.
Further, in the case of the white LED which is formed by combining a light-emitting device, such as a nitride-based compound semiconductor capable of emitting an ultraviolet light, and a fluorescent material emitting light in response to the excitation by the ultraviolet light, this white LED has been reputed as incurring difficulty in obtaining a white color of high luminance because the luminous efficiency of the fluorescent material of the red component is appreciably low as compared with that of any other fluorescent material and consequently the mixing ratio of this fluorescent material is proportionately large.
This invention is aimed at providing an oxynitride-based fluorescent material which permits impartation of enhanced luminance to a white light-emitting diode (white LED) using a blue light-emitting diode (blue LED) or an ultraviolet light-emitting diode (ultraviolet LED) as the light source and a light-emitting device using the fluorescent material.
The present inventor has pursued a diligent study with a view to accomplishing the object mentioned above and has consequently found that an oxynitride-based fluorescent material resulting from substituting Eu for part of M in the general formula 2MO.Si3N4 (wherein M denotes one or more elements selected from among Be, Mg, Ca, Sr and Ba) possesses a strong absorption band in a wide range extending over ultraviolet to near ultraviolet to visible light. He has found anew that the white LED using the fluorescent material of this invention possesses an excellent light-emitting property. This invention has been perfected as a result.